1. Field of the Invention
The present invention relates to a data transmission system, and more particularly to a method of and an apparatus for data transmission in an ATM (Asynchronous Transfer Mode) communication system according to defined standard B-ISDN (Broad Integral Service Digital Network).
2. Description of the Related Art
According to one conventional data transmission system, as shown in FIG. 1 of the accompanying drawings, a receiver has an error detecting circuit 8 for detecting the number of error bits and an error correcting circuit group 7 which is controlled by an optimum error correcting encoding circuit that is selected by the error detecting circuit 8 based on the detected number of error bits. Error detecting circuit 8 transmits information representing the selected optimum error correcting encoding circuit to a transmitter for controlling error correcting encoding circuit group 6, in a mode to coordinate with the receiver. For details of such a conventional data transfer system, reference should be made to Japanese laid-open patent publication No. 159421/1991.
FIG. 2 shows another conventional data transmission system. In FIG. 2, when a data error is detected by error detecting circuit 11 of a receiver, retransmission request output circuit 12 of the receiver outputs and transmits a retransmission request to a transmitter. The number of produced retransmission requests is counted by counters 10T, 10R provided in the transmitter and receiver respectively. Based on the count, error correcting encoding circuit group 9 of the transmitter and error correcting circuit group 13 of the receiver are controlled in coordination with each other. For details of such a conventional data transfer system, reference should be made to Japanese patent laid-open publications Nos. 119135/1989 and 119136/1989.
In an ATM network available for B-ISDN, an error rate and a state of congestion in the network are recognized by a transmitter of a transmitting apparatus based on network management information that is represented by information obtained from OAM (operation and maintenance) functions prescribed by (International Telecommunication Union-Telecommunications Sector) Recommendations.
If the network is determined as being not congested, then data are transmitted without an error correcting encoding process, so that the data can be transmitted with an increased transmission efficiency at an increased transmission rate by avoiding redundancy bits and a transmission delay that would be introduced by an error correcting encoding process. If the network is determined as being congested or suffering many data errors, then depending on the degree to which the network is congested or suffering data errors, the data are subjected to either an error correcting encoding process for bit errors or a powerful error correcting encoding process for a cell loss while sacrificing transmission efficiency. Therefore, depending on the contents of the data to be transmitted or the application to which the data are used, it is possible for a transmitter to effect an optimum control operation, operating a receiver as a slave to the control of the transmitter.
However, the above data transmission process cannot be realized by the conventional data transfer systems shown in FIGS. 1 and 2 in which the quality of transmitted data is monitored by the receiver and the receiver controls the error correcting encoding circuit of the transmitter. Another problem of the conventional data transfer systems is that the above data transfer process cannot be carried out by one-way communications because control information of the receiver cannot be transmitted by one-way communications.